4 State Environmental Protection Key Laboratory of Sources and Control of Air Pollution Complex, School of Environment, Tsinghua University, Beijing 100084, China

Highlights

The extinction was discussed over four seasons and different AQI levels.(NH4)2SO4 and NH4NO3 are the main contributors to bext.The reduction of SNA could be more effective in improving visibility.

Abstract

Daily PM2.5 samples were collected in Beijing across four consecutive seasons from June 2012 to April 2013. Major water-soluble inorganic ions, carbonaceous species and elements were analyzed to investigate their temporal variations and evaluate their contributions to visibility impairment over different seasons and under different pollution levels. The mass concentrations of PM2.5 ranged from 4.3 to 592.4 µg m–3, with an annual average of 112.4 ± 94.4 µg m–3. The predominant components of PM2.5 were secondary inorganic ions (NH4+, NO3– and SO42–) and carbonaceous compounds, which accounted for 45.9% and 24.1% of the total PM2.5 mass, respectively. Distinct seasonal variation was observed in the mass concentrations and chemical components of PM2.5. The average mass concentrations of PM2.5 were the highest in winter, followed by spring, and lowest in autumn. Light extinction coefﬁcients (bext) were discussed over four seasons. (NH4)2SO4 was the largest contributor (28.8%) to bext, followed by NH4NO3 (24.4%), organic matter (19.5%), elemental carbon (7.4%), and coarse mass (7.2%), while fine soil, sea salt, NO2 and Rayleigh made minor contributions, together accounting for 12.7% of bext. During the polluted periods, the contributions of (NH4)2SO4 and NH4NO3 to bext increased dramatically. Therefore, in addition to control primary particulate emissions, the reduction of their precursors like SO2, NOx and NH3 could effectively improve air quality and visibility in Beijing.